As the information and communication technology is rapidly developed, the communication technology for mass signal transmission at high speed is also rapidly developed. Specifically, as the demand of mobile phones, satellite communications, radar devices and the like is progressively expanded in the wireless communication technology, there are increased the requirements for high-speed and high-power electronic devices to be used for the super-high speed information and communication systems of microwave or millimeter broadband.
As an example of a compound semiconductor, since gallium nitride (GaN) has the physical properties, such as high energy band gap, high thermal and chemical stability, high electron saturation velocity and the like, it is applicable to a high-frequency and high-power electron device as well as an optical device. Accordingly, much research of gallium nitride is actively conducted in many fields. A gallium nitride thin film has the various advantages, such as high breakdown field, maximum current density, stable high-temperature operation, high thermal conductivity and the like. In the case of a device (for example, HFET, HEMT and others) using a heterojunction structure of AlGaN/GaN, since the band-discontinuity in an junction interface is great, electrons in high concentration are drifted in the interface, to more increase electron mobility. Further, since the gallium nitride thin film has high velocity of surface acoustic wave (SAW) and excellent temperature stability and produces the effect of polarization of piezoelectric properties, it can be easily manufactured as a band pass filter which is capable of operating at □ or more.
The gallium nitride thin film can be grown on a sapphire substrate by generally using the metal-organic CVD method or the molecular beam epitaxy (MBE) method. Then, since the sapphire substrate and the gallium nitride thin film are greatly different from each other in the respective lattice constant and the coefficient of thermal expansion, it is very difficult to grow a single crystal and many defects occur in growing the thin film. When gallium nitride is grown at high temperature of 800° C. or above, since it naturally has n-type conductive properties by the nitrogen vacancy which is formed due to high volatility of nitrogen and by the influence of the impurity, such as oxygen, it is very difficult to grow a semi-insulating layer thin film. Therefore, when manufacturing the electronic devices, such as HFET and SAW filters, leakage current occurs so that low trans-conductance and insertion lose may be caused.
In conventional technology for manufacturing a semi-insulating thin film substrate, a method of doping the substrate with an n-type dopant and a p-type dopant simultaneously may be used to compensate electrical properties of the substrate. In the method of doping the p-type and n-type dopants simultaneously, excessive counter-doping is performed for the electrical compensation. Then, carrier mobility may deteriorate.
Further, to actually apply a gallium nitride substrate as a light-emitting device, an electronic device or others, a doping process for giving electrical properties is accompanied. The n-type or p-type dopant being implanted onto the gallium nitride substrate provides conductive properties to gallium nitride which is electrically neutral, to enable various optical and electronic operations.
Charge concentration is an important factor to determine the electrical properties of gallium nitride. Since the electrical conductance changes from the insulating properties to the conductive properties according to the charge concentration, the application field of gallium nitride may vary as the properties change. There are two methods of controlling n-type charge concentration within a gallium nitride wafer. One is to control the charge concentration by controlling an amount of a dopant to be actually implanted, and the other is to control the charge concentration by simultaneously implanting the n-type dopant and the p-type dopant so that an electron from the n-type dopant and a hole from the p-type dopant are compensated for each other.
In the former, it is widely used as a method of controlling the charge concentration but it has the drawback that an activation ratio of the dopant decreases as the amount of the dopant increases. In manufacturing the n-type gallium nitride substrate of high concentration, when an excessive amount of the dopant is implanted to overcome the low activation ratio, crystalline properties deteriorate and cracks may be caused. In the latter, it has the drawback that the charge concentration rapidly decreases, and it needs to perform the dual process which simultaneously satisfy the process conditions of implanting the n-type dopant and of implanting the p-type dopant.
In growing gallium nitride bulk, the extent of activation of the dopant to act as a charge within the wafer and the extent of efficient control of the activation ratio are very important in determining the electrical properties of gallium nitride. However, any effective plans for controlling the charge concentration by controlling the activation ratio of the charge have not yet been proposed. Therefore, it needs a method of effectively controlling electrical properties, which can be broadly applied to a compound semiconductor, such as gallium nitride.